ScatterSeries marker shapes: brushFilename seems to be ignored?

I'm trying to use different-shaped markers in a ChartView Scatter Series (2D), and I can see that there is a brushFilename option for te ScatterSeries that suggests it might be used to define the shape of the markers. There is very little documentation or example, except one C++ example that creates a brush to be used (https://doc.qt.io/qt-5/qtcharts-scatterchart-example.html)
That's not very helpful because it doesn't use a file as QML seems to expect, but it does suggest that what I want is possible...

So, what kind of file does brushFilename expect?
I've assumed SVG at the moment so that it can be coloured easily. I also noted that the C++ example initially sets the marker as a rectangle before setting the marker using the painted image, so I tried doing that in the QML, but all I get now is a rectangle instead of a circle.